Molecular chaperones are essential components of the machinery facilitating import of nuclear-encoded proteins into the mitochondria. They act at several steps of the complex import pathway. Cytosolic hsp 70 appears to contribute to maintaining precursors in a translocation-competent conformation. Mitochondrial hsp 70 has a distinct role in driving translocation across outer and inner mitochondrial membranes and probably in supporting unfolding of precursors in the cytosol. Hsp 60 in the matrix is involved in facilitating folding and assembly of imported polypeptide chains.